There is known a conventional fixing apparatus of this type as disclosed in the under-listed Patent Document 1. This fixing apparatus comprises an abutment member whose one and the other ends are abutted with the side surfaces of two structural members respectively, first and second engagement members arranged between this abutment member and the two structural members, a female screw member disposed at the first and second engagement members such that one and the other ends of the female screw member are movable in the widthwise direction of the engagement groove but non-movable in the direction away from the structural members, and a male screw member passed through the abutment member and threadingly engaged with the female screw member. Two side parts adjacent to the two structural members of the first engagement member and two side parts adjacent to the two structural members of the second engagement members are each provided with an engagement part.
To fixing two structural members by a fixing apparatus thus constructed, first, the first and second engagement members are moved toward each other so that their engagement parts are inserted into the engagement grooves through between protrusions formed on the open side end parts of two side wall surfaces of the engagement grooves. Thereafter, the male screw member passed through the abutment member and threadingly engaged with the female screw member is tightened. Then, the first and second engagement members are moved away from each other and the engagement parts of the engagement members are brought into opposing relation with the protrusions of the engagement grooves. Thereafter, when the male screw members is tightened further, two ends of the abutment member are abutted with the structural members respectively and the engagement parts of the first and second engagement members are abutted with the protrusions of the engagement grooves of the two structural members respectively. By this, the pair of structural members are fixed to each other.    Patent Document 1: Japanese Patent Publication No. 3,365,629